Small, battery-operated electrical appliances are typically charged at an external charging station. Contactless charging stations that inductively transfer electrical energy from the charging station to the appliance are especially popular. For this purpose, an alternating magnetic field is generated in the charging station by an oscillator that has a coil element and a capacitor element, wherein the coil element simultaneously forms the primarily coil of an inductive transformer and the secondary coil of the transformer is arranged in the appliance to be charged. Therefore the charging station is designated as the primary side and the appliance to be charged is designated as the secondary side. Such a charging station in which the oscillator is operated with a stabilized voltage or oscillates with a constant amplitude is known from JP 6-54454 A.
Modern charging stations typically have three operating states. The first state is the operating mode in which the secondary side continuously requests power, for example to operate the appliance or to charge a battery installed in the appliance. The second state is a simple standby mode in which the appliance is not located in the charging station, i.e. no power at all is requested. The third state is what is known as extended standby mode in which the appliance is located in the charging station but only occasionally requires power, for example, because although the battery is fully charged, it must occasionally be recharged to compensate for self-discharge or the power consumption of the appliance. In the latter case, the charging station should switch back and forth between the simple standby mode and the operating mode as needed. The respective operating state of the charging station (primary side) is thus determined by the energy demand of the small electrical appliance (secondary side).
It is known to detect the energy demand of the secondary side directly at the secondary side, to transfer corresponding information to the primary side and to adjust the oscillator accordingly, for example to adjust the base emitter voltage of a transistor operating in the oscillator. This solution is quite complicated because transmission means for the information from the secondary side to the primary side are required. Alternatively, the energy demand of the secondary side could be determined in that the (primary side) power consumption of the oscillator is measured and the oscillator is controlled accordingly. However, this variant is less suitable for adjusting multiple operating states because, due to the typically weak coupling between the primary and the secondary sides of the transformer, the power consumption of the charging station is only weakly affected by the power consumption of the appliance.
Thus, it would be desirable to minimize the power consumption of the charging station in a standby mode for reasons of energy efficiency. It would also be desirable to specify a method for inductive energy transfer and a circuit arrangement that can be placed into an operating state with further reduced power consumption in a simple manner depending on the power requirement of the secondary side.